At this time high bulk paper is produced commercially using through-air drying processes developed more than a decade ago. One of the first patents on this subject was that of Sanford et al (U.S. Pat. No. 3,301,746 issued Jan. 31, 1967 and assigned to The Procter and Gamble Co.), and this was followed by several patents, among them that of Shaw (U.S. Pat. No. 3,821,068 issued June 28, 1974 and assigned to Scott Paper Company).
Through-air drying produces high bulk ligno-cellulosic fibre webs by avoiding the application of compressive forces to the formed paper web on the forming wire or in the press-section until such time as the paper is essentially dry, at which stage the compressive forces used for sheet-transferring and pressing can be applied without substantial loss of bulk. The web is then further dried on a conventional Yankee dryer creped.
In general, through-air drying gives a relatively low-strength sheet, and the various commercial processes differ mainly in the methods used to strengthen the product. Some claim to rely on natural interfibre bonding forces for strength, while others use selective densification or an adhesive bonding system.
Other processes claimed for production of high bulk paper are as follows:
Gatward et al (U.S. Pat. No. 3,716,449 issued Feb. 13, 1973 and assigned to Wiggins Teape Research and Development) form paper webs from a thixotropic foam; and
Lesas et al (U.S. Pat. No. 4,204,054 issued May 20, 1980 and assigned to Beghin-Say) use chemically modified fibres to attain the desired high bulk paper products.
The major objective of all of the above processes is to produce a soft, bulky, highly absorbent paper for the manufacture of sanitary tissue products.
The term "high bulk paper" generally means a creped paper having a density between 0.10 and 0.17 g/cm.sup.3, based on thickness measured by a caliper gauge at 42.2 g/cm.sup.2 pressure with an anvil area of 6.45 cm.sup.2.
In spite of claims to the contrary, for a given basis weight, the unit tensile strength of most high bulk tissues is lower than that of conventional tissues. Nevertheless, the important features of high bulk, softness and good absorbency are attained, and consumer acceptance is excellent. In addition, manufacturing economies are achieved because, due to the high bulk (low density) of the sheet, a given area of tissue (and a given volume or roll diameter of tissues) can be created from fewer tonnes of raw material (fibres).
A severe drawback of the presently used commercial processes for production of high bulk paper which use through-air drying is the excessive amount of energy required to achieve water removal by hot-air drying compared with conventional press removal of water. The invention described herein has the advantage of producing high bulk, soft and absorbent paper products without the expenditure of large amounts of energy to remove the moisture from the web.
The invention makes use of the fundamental nature or behavior of ligno-cellulosic fibres. Ligno-cellulosic fibres are stiff, elastic, and springy in the dry or substantially dry condition (say 70-100% solids), and quite the opposite in the fully wetted hydrated state (say 35 to 45% solids). The hydration of papermaking fibres is the very base of conventional papermaking, involving wetting of the cell walls to make them pliable and conformable so as to be able to create the interfibre or papermaking hydrogen bond. The processes of pulping and wet refining are generally the steps used to hydrate the fibres and render them suitable for formation of interfibre bonds. After these steps, the compressive forces of presses act upon the papermaking fibres to remove the water from the paper web and bring the fibre into close proximity to each other. The fibres then remain in this position until papermaking hydrogen bonds are formed by the so-called Campbell forces of the receding meniscus of the water layer between adjacent fibres. In this specification the term "dry fibres" will be used to describe fibres having more than 70% solids, and the term "hydrated" will be used for ligno-cellulosic fibres which have been sufficiently wetted to become papermaking bond forming fibres. The figures for solids content used herein refers to the solids content of the fibre walls.
As stated above, in the dry or substantially dry condition, ligno-cellulosic fibres are stiff, elastic and springy, and when compressed only partially conform to each other. As soon as pressure is released they partially regain their original shape and break their proximity from nearby fibres. Under these conditions the papermaking bond cannot be effectively formed by the above described Campbell forces. This is the essence of the through-air drying prior art by which the fibre web can be compressed without loss of bulk only after sufficient dryness has been attained via through-air drying.
An objective of this invention is to reduce the cost of manufacturing high bulk tissues as compared to processes using through-air drying by using conventional pressing for major water removal.
Another objective of this process is to avoid the introduction of any potentially harmful chemicals into the paper, by using natural unmodified ligno-cellulosic fibres.
Another objective is to avoid the use of specially designed paper machines e.g., based on through-air drying or foam forming.
The invention allows a range of tissue products to be produced having qualities of high bulk, softness and absorbency similar to those of high bulk tissue, and having sufficient strength for converting and consumer use.